In recent times, a sizable industry has developed for the packaging of electronic components, such as micro-electronic chips and integrated circuits. Such packaging has as one of its objectives to prevent electrostatic discharge from damaging or destroying these components. The envelopes or bags in commercial use are known as anti-static packages, which generally consist of:
1. An inner layer or ply of an anti-static polyethylene of between 1 to 4 mils thickness, that has a surface resistivity of between 10.sup.8 -10.sup.13 ohms per square to minimize triboelectric charge build-up and avoid electrical damage.
2. A dielectric or non-conductive layer may be adhered to the anti-static layer to provide electrical insulation, over which a conductive or metallized layer of aluminum or nickel is applied. The metal provides a conductive ground plane, and is applied in a very thin layer, usually by vacuum deposition, between 1 and 150 angstroms thick , in order to provide partial transparency.
In order to protect the thin metallized layer, an abrasion-resistant layer is applied over the thin metal layer.
In one approach, a resinous material is used for this abrasion layer, which is applied in ultra-thin thickness of about 0.15 micrometers. Such a coating is taught in U.S. Pat. Nos. 3,118,781 and 4,154,344. The resinous coating comprises a copolyester of equal parts terephthalic acid and isophthalic acid with ethylene glycol, which is applied from a solution of cyclohexanone at a solids concentration of 1.5 percent by weight.
Such a coating, while claiming to be an abrasion resistant layer, in practice has been known to "crumb off" with the metallized conductive layer and which could contaminate components and clean rooms. In addition, this layer has a surface resistivity of less than 5.times.10.sup.4 ohms per square. Because of this low resistivity, this layer will bleed off to the conductive layer, discharging current at a high rate. This has the potential to produce arcing through the dielectric shield to "protected" components, and is a shock hazard to personnel.
In another approach, as taught in U.S. Pat. No. 4,424,900, the abrasion-resistant coating is an insulating layer of polyester of about 0.5 to 2.0 mils thick. The volume resistivity of 10.sup.10 ohms per centimeter for this layer translates to a minimum surface resistivity of 10.sup.13 ohms per square.
This approach will not dissipate electrostatic charges and therefore does not provide protection against high voltage exposure of about 12 KVA, and is likely to short out by arcing.
By contrast, the coating of this invention has a demonstrated ability to withstand multiple repetitive charges without capacitive storage. Application of voltage of 12 KVA is sustained for periods of up to 150 seconds. The electrical stability of this polymer not only provides a controlled rate of discharge, but maintains this protection over the functional life of the package (envelope).